A known sewing machine such as illustrated in FIG. 15 is disclosed in JP08-309062A (hereinafter referred to as Reference 1), for example. Specifically, a sewing machine 100 disclosed in Reference 1, and illustrated in FIG. 15 includes a base frame 101 to which a thread tensioner 110 (a needle thread tension control device) is fixed by a screw 103. A portion of an operation dial 117 of the thread tensioner 110 projects from an opening 102a of an external cover 102 of the sewing machine 100.
FIG. 16 is an explanatory view of the thread tensioner 110 illustrated in FIG. 15. As illustrated in FIG. 16, one end of a thread tension shaft 112 including a lead screw portion 112a is fixed to a thread tension bracket 111 of the thread tensioner 110. A pair of tension discs 113, 113 sandwiching and holding a needle thread so as to apply a tensile force (a tension), a tension spring 114, a spring retainer 115, a pressing spring 116, and the operation dial 117 are assembled from the other end of the thread tension shaft 112 so as to be positioned at the thread tension shaft 112 in the aforementioned order from the one end to the other end of the thread tension shaft 112. A stopper 118 is provided to block the operation dial 117 from disengaging from the thread tension shaft 112 at a time when the sewing machine 100 is assembled. The tension spring 114 applies a spring load to the pair of tension discs 113, 113 while the pressing spring 116 applies a spring load to the operation dial 117. The operation dial 117 includes two cylindrical portions coaxially arranged to the thread tension shaft 112. One of the cylindrical portions positioned at an inner side includes a groove 117a in parallel to the thread tension shaft 112. An outer peripheral end portion 115a of the spring retainer 115 engages with the groove 117a. Numeric values indicating a needle thread tension setting level are illustrated on an outer peripheral surface 117p of the operation dial 117. In a case where a user of the sewing machine 100 rotates the operation dial 117 within one rotation range thereof, the spring retainer 115 rotates in association with the operation dial 117 while sliding in an axial direction of the thread tension shaft 112 relative to the operation dial 117. As a result, a distance between the pair of tension discs 113, 113 and the spring retainer 115 changes, which causes a change in a pressing load applied to the pair of tension discs 113, 113. The needle thread tension is adjusted and controlled accordingly. According to the sewing machine 100 equipped with the thread tensioner 110, in a state where the external cover 102 is attached to the base frame 101, the portion of the operation dial 117 projects from the opening 102a of the external cover 102. In addition, an end surface 117b of the operation dial 117 arranged at a right side in FIG. 16 is in contact with an end surface 102b of the opening 102a arranged at the right side.
Further, JP07-284584A (hereinafter referred to as Reference 2) discloses another thread tensioner where a display of a scale is changeable by a simple rotation of a ring, without an actual change of the needle thread tension, in a state where an operation dial is fixed. According to the thread tensioner disclosed in Reference 2, the display of the scale should fully appear on an outer periphery of the ring corresponding to one rotation angle thereof.
According to the thread tensioner 110 disclosed in Reference 1, the numeric values indicating the needle thread tension setting level appear on the outer peripheral surface 117p of the operation dial 117 in a circumferential direction thereof, instead of a spiral direction. That is, the display of the scale indicating the needle thread tension is limited within a range corresponding to one rotation of the operation dial 117 about an axis thereof. That is, a distance from a start point to an end point of the scale is limited within a range corresponding to 360° of the operation dial 117 on the outer peripheral surface 117p. As a result, the distance from the start point (a maximum tensile value) to the end point (a minimum tensile value) of the scale is inhibited from being elongated. In this case, the scale indicating the needle thread tension is inhibited from being finely specified. Further, because the display of the scale is limited within the range corresponding to one rotation of the operation dial 117 about the axis thereof, the maximum tensile value may be inhibited to increase. Even in such case, an increase of an outer diameter of the operation dial 117 achieves an increase of a peripheral length of the outer peripheral surface 117p of the operation dial 117, which leads to an increase of the distance from the start point to the end point of the scale. However, an excessive enlargement of the outer diameter of the operation dial 117 may occur.
According to the thread tensioner disclosed in Reference 2, the scale indicating the needle thread tension should fully appear on the outer periphery of the ring corresponding to one rotation angle thereof. Therefore, a distance from a start point (a maximum tension value) to an end point (a minimum tension value) of the scale is limited within a range corresponding to 360° of the ring and is inhibited from being elongated. The scale indicating the needle thread tension is inhibited from being finely specified.
A need thus exists for a sewing machine which is not susceptible to the drawback mentioned above.